Transmembrane transport of nutrient molecules is a critical cellular function. Because practitioners have recognized the importance of transmembrane transport to many areas of medical and biological science, including drug therapy and gene transfer, there has been significant research efforts directed to the understanding and application of such processes. Thus, for example, transmembrane delivery of nucleic acids has been encouraged through the use of protein carriers, antibody carriers, liposomal delivery systems, electropotation, direct injection, cell fusion, viral carriers, osmotic shock, and calcium-phosphate mediated transformation. However, many of those techniques are limited both by the types of cells in which transmembrane transport is enabled and by the conditions of use for successful transmembrane transport of exogenous molecular species. Further, many of these known techniques are limited in the type and size of exogenous molecule that can be transported across a membrane without loss of bioactivity.
One method for transmembrane delivery of exogenous molecules having a wide applicability is based on the mechanism of receptor mediated endocytotic activity. Unlike many other methods, receptor mediated endocytotic activity can be used successfully both in vivo and in vitro. Receptor mediated endocytosis involves the movement of ligands bound to membrane receptors into the interior of an area bounded by the membrane through invagination of the membrane. The process is initiated or activated by the binding of a receptor specific ligand to the receptor. Many receptor mediated endocytotic systems have been characterized, including those recognizing galactose, mannose, mannose 6-phosphate, transferrin, asialoglycoprotein, transcobalamin (vitamin B.sub.12), .alpha.-2 macroglobulins, insulin, and other peptide growth factors such as epidermal growth factor (EGF).
Receptor mediated endocytotic activity has been utilized for delivering exogenous molecules such as proteins and nucleic acids to cells. Generally, a specified ligand is chemically conjugated by covalent, ionic or hydrogen bonding to an exogenous molecule of interest, (i.e., the exogenous compound) forming a conjugate molecule having a moiety (the ligand portion) that is still recognized in the conjugate by a target receptor. Using this technique the phototoxic protein psoralen has been conjugated to insulin and internalized by the insulin receptor endocytotic pathway (Gasparro, Biochem. Biophys. Res. Comm. 141(2), pp. 502-509, Dec. 15, 1986); the hepatocyte specific receptor for galactose terminal asialoglycoproteins has been utilized for the hepatocyte-specific transmembrane delivery of asialoorosomucoid-poly-L-lysine non-covalently complexed to a DNA plasmid (Wu, G. Y., J. Biol. Chem., 262(10), pp. 4429-4432, 1987); the cell receptor for epidermal growth factor has been utilized to deliver polynucleotides covalently linked to EGF to the cell interior (Myers, European Patent Application 86810614.7, published Jun. 6, 1988); the intestinally situated cellular receptor for the organometallic vitamin B.sub.12 -intrinsic factor complex has been used to mediate delivery to the circulatory system of a vertebrate host a drug, hormone, bioactive peptide or immunogen complexed with vitamin B.sub.12 and delivered to the intestine through oral administration (Russell-Jones et al., European patent Application 86307849.9, published Apr. 29, 1987); the mannose-6-phosphate receptor has been used to deliver low density lipoproteins to cells (Murray, G. J. and Neville, D. M., Jr., J. Bio. Chem, Vol. 255 (24), pp. 1194-11948, 1980); the cholera toxin binding subunit receptor has been used to deliver insulin to cells lacking insulin receptors (Roth and Maddox, J. Cell. Phys. Vol. 115, p. 151, 1983); and the human chorionic gonadotropin receptor has been employed to deliver a ricin a-chain coupled to HCG to cells with the appropriate HCG receptor in order to kill the cells (Oeltmann and Heath, J. Biol. Chem, vol. 254, p. 1028 (1979)).
The method of the present invention enhances the transmembrane transport of an exogenous molecule across a membrane having biotin or folate receptors that initiate transmembrane transport of receptor bound species. The method takes advantage of (1) the location and multiplicity of biotin and folate receptors on the membrane surfaces of most cells and (2) the associated receptor mediated transmembrane processes. Performance of the method involves formation of a complex between a ligand selected from biotin or other biotin receptor-binding compounds, and/or folic acid or other folate receptor-binding compounds, and an exogenous molecule. A cell membrane bearing biotin or folate receptors is contacted with this complex, thereby initiating receptor mediated transmembrane transport of the complex. The complex is allowed to contact the membrane surface bearing the corresponding receptors for a time sufficient to initiate and permit transmembrane transport of the complex. The transmembrane transport of exogenous molecules including proteins and polynucleotides has been promoted in plant, mammalian, and bacterial cells.
In one embodiment of this invention, the target receptor for the method of the present invention is the biotin receptor. Biotin is a necessary cellular nutrient that has been found to be preferentially bound by biotin receptor proteins associated with cellular membranes. Commercially available reagents are used to form a covalent complex between biotin and polynucleotides, proteins, or other desired exogenous molecules. According to one preferred embodiment of the present invention, a biotin/exogenous molecule complex is brought into contact with a membrane having associated biotin receptors for a time sufficient to allow binding of the biotin moiety of the complex to a corresponding biotin receptor in the membrane. This binding triggers the initiation of cellular processes that result in transmembrane transport of the complex.
In an alternate but equally preferred embodiment of this invention, folate receptors are targeted to enhance cellular uptake of exogenous molecules. Folate binding receptors are found in most types of cells, and they have been demonstrated to bind and trigger cellular internalization of folates. Thus, folic acid and other art-recognized folate receptor-binding ligands can be chemically bonded to polynucleotides, proteins, or other desired exogenous molecules using art-recognized coupling techniques to provide a folate receptor-binding complex which is readily endocytosed into living cells. In accordance with this embodiment of the present invention, a folate/exogenous molecule complex is brought into contact with a membrane having associated folate receptors for a time sufficient to allow binding of the folate moiety of the complex to a corresponding folate receptor. Folate receptor-binding triggers the initiation of cellular processes that result in transmembrane transport of the complex.
The methods of this invention are particularly useful for increasing the internalization efficiency (cellular uptake) of exogenous molecules that are normally resistant to cellular internalization. Proteins and polynucleotides previously recognized as difficult to move across cell membranes can be internalized by a cell through application of the method of the present invention. For example, transformation of target cell lines resulting in expression of a protein product has been accomplished by coupling the desired polynucleotide to either biotin or folates, and contacting the cells with the resulting complex for a time sufficient to promote cellular internalization. In one case, a DNA plasmid containing a gene sequence coding for chloramphenicol acetyltransferase (CAT), was biotinylated and transported into E. coli via a biotin receptor mediated endocytotic pathway and expressed. Similar examples of transformation or transfection have been noted for biotin or folate linked nucleic acids in mammalian systems, prokaryotic systems, and plants. The use of biotin and folates complexes to enhance cellular uptake of complexed exogenous molecules has been demonstrated in vivo and in vitro.